Apparatuses that manage video or audio contents as digital data are now being widely used, and correspondingly, TV broadcast also becomes increasingly digitalized. The more increased number of channels can be provided in digital broadcasting as compared to analog broadcasting. Further, program information or the like distributed together with video or audio signals can provide a variety of services.
For example, some of the receivers that can receive a digital satellite broadcast wave, digital terrestrial broadcast wave, or the like, include a plurality of tuner circuits, and make them work at a time to allow a viewer to enjoy TV programs timely as normal. Further, these receivers can store the TV pictures in a random-accessible recording medium such as an HDD (Hard Disc Drive), to reproduce, as needed, the immediately preceding TV pictures even while enjoying TV programs.
In this case, mutual interference in high frequency range is caused due to simultaneous use of the tuner circuits. In other words, the tuner circuits are in a state of receiving signals, from parabolic antennas independent from each other, having the same frequency yet differing in level. At this time, high-frequency current flows in respective tuner circuits, around which electric field or magnetic field is caused by radiated electromagnetic wave.
Since the tuners are attached to the back panel side of a so-called set-top box (STB) in general, sufficient space cannot be assured between the tuners in many cases. Accordingly, magnetic field or electric field generated from the respective tuner circuits affects other tuner circuits, which may cause serious electromagnetic interference such as cochannel interference.
Concrete configurations of tuner circuits 51 and 52 of this type are shown respectively in FIG. 1. A base plate 61 that constitutes each of the tuner circuits 51 and 52 is designed according to a so-called microstripline structure using a mount layer 62 and a ground layer 63. Thus, quantity of high frequency current flows on the side of the mount layer 62. In the above configuration, particularly, as shown in FIG. 1, when the two tuner circuits 51 and 52 are arranged adjacently in parallel, electromagnetic wave caused by high frequency current is radiated from a circuit group 64 on the mount layer 62 of the tuner circuit 51 to the tuner circuit 52 arranged next to the tuner circuit 51. To the ground layer 63, high frequency current flows through a capacity-coupled dielectric layer 65.
Note that, in FIG. 1, reference numerals 61 and 62 are input terminals of the tuner circuits 51 and 52.
To avoid the above problem, a cable is disposed within the set-top box to form a predetermined space between the tuner units, or shield cases 66 and 67 obtained by processing a metal are used to tightly cover the tuner units to shield electric field or magnetic field.
As shown, for example, in FIG. 2, a configuration that incorporates two receiving system and a device for storing TV programs is being considered as the receiver for digital satellite broadcasting services. This receiver receives a digital satellite broadcast from a first antenna 71, and then down-converts it to an intermediate frequency signal. The intermediate frequency signal is supplied to a first tuner 81 of a receiver 80, followed by being converted into a baseband signal and subjected to processing such as demodulation and error correction. Then, the resultant signal is output as a first TS (Transport Stream), which is then supplied to a switching circuit 83.
A second digital satellite broadcast is received by means of a second antenna 72, and then down-converted to an intermediate frequency signal. The intermediate frequency signal is supplied to a second tuner 82. The second tuner 82 then applies the same processing as the first tuner 81 to the intermediate frequency signal to output a second TS, which is then supplied to the switching circuit 83.
An HDD 84 serving as a device for storing TV programs is connected to the switching circuit 83. Either the TS output from the first tuner 81 or the TS output from the second tuner 82 is recorded (stored) in the HDD 84 through the user's operation.
Any one of the TSs output from the first tuner 81 and the second tuner 82, and the TSs read out from the HDD 84 is selected and taken through the user's operation. The taken TS is then supplied to an MPEG decoder 85, followed by being decoded into a digital video signal and digital audio signal.
The decoded digital video signal is supplied, for example, to an NTSC encoder circuit 86 and encoded into a digital video signal according to the NTSC specification. The resultant signal is then supplied to a D/A converter circuit 87V and D/A converted into an analog NTSC video signal, followed by being output from an output terminal 88V. The digital audio signal decoded by the MPEG decoder 85 is supplied to a D/A converter 87S, and D/A converted into an original analog audio signal, followed by being output from an output terminal 88S.
A system control circuit 89 has a microcomputer and supplies control signals to the respective circuits through the user's operation.
Each of the first and second tuners 81 and 82 has the configuration as shown in, for example, FIGS. 3 and 4. That is, the first and second tuners 81 and 82 have the same configuration, in which components 92 and 93 including an IC and the like are mounted on both surfaces of a double-sided printed board 91 having a rectangular shape. Frame-like shield plates 94 and 95 are so disposed as to surround high frequency portion of the printed board 91, and are covered by plate-like shied covers 96 and 97. In FIG. 3, the shied cover 96 has been removed.
An antenna input connector 98 and a connector pin 99 are attached in parallel with the printed board 91 to one narrow side and one wide side of the printed board 91, respectively.
The above configured first and second tuners 81 and 82 are mounted, in parallel with each other, on a main printed board 302 such that the two connectors 98 and 98 project through a rear-end panel 301 of the receiver 80, as shown in FIG. 4. Attached to the printed board 302 are two connectors (not shown) into which the connector pins 99 and 99 of the first and second tuners 81 and 82 are inserted, thereby establishing physical connection between the first and second tuners 81 and 82, and printed board 302.
With this receiver 80 in which the video and audio signals of a TV program received by one of the first and second tuners 81 and 82 are output to the output terminals 88V and 88S, the user can enjoy TV broadcast. At the same time, it is possible for the user to record the TV program that he or she is watching onto the HDD 84, or to record the program on a different channel that has been received by the other one of the first and second tuners 81 and 82 onto the HDD 84. Further, the user can enjoy the programs that have been recorded onto the HDD 84.
Shielding of the tuner units using the shield case or the like as described above increases the number of components, which may lead to an increased product cost. Further, in order to provide the shield case, additional space becomes necessary between the base plates. This may prevent effective arrangement of the tuner units in the set-top box.
Further, when the tuner units are covered by the shield cases, electric field and magnetic field generated in the space within the shield case allow high frequency current to flow on the case surface. The high frequency current that has been propagated through the shield case may affect the other tuner circuit depending on the arrangement of the tuner units, which may cause mutual interference.
In the first and second tuners 81 and 82 configured as shown in FIGS. 3 and 4, when the first tuner 81 and the second tuner 82 are adjacently arranged, unnecessary radiation from the first tuner 81 goes into the second tuner 82, and conversely, unnecessary radiation from the second tuner 82 goes into the first tuner 81 to thereby generate mutual interference. As a result, the bit error rates or the like of the TSs output from both the tuners 81 and 82 are increased to deteriorate the performance.
Therefore, in each of the first and second tuners 81 and 82, the high frequency portion thereof is sealed by the shield plates 94 and 95, and the shield covers 96 and 97, thereby preventing mutual interference even when the first and second tuners 81 and 82 are adjacently arranged.
When the two sets of the shield plates 94, 95 and the shield covers 96, 97 are provided for the first and second tuners 81 and 82, respectively, the number of shield members is increased, with the result that the total number of the components is increased to lead to high product cost.
The mutual interference between the first and second tuners 81 and 82 can be reduced by arranging them on the main printed board 302 so as to be spaced sufficiently apart from each other. However, when the above arrangement is employed, the interval between the two connectors 98 and 98 is increased, not only to lower operating efficiency of the receiver 80, but to make the design of the appearance thereof impractical.